Session 4:

Polymerization of
Substituted Olefins

Question:

Besides polyethylene, what other
kinds of polyolefin polymers are possible?

Text:

While polyethylene has proven to be an
extremely versatile and useful plastic, its properties are not
ideal for all applications. For this reason, chemists have
investigated the polymerization of other olefin monomers,
particularly monomers that possess a substituent group other than
hydrogen on one of the olefinic carbon atoms. The polymers that
result from these reactions do, in fact, possess different
physical properties from those of LPDE and HDPE and have found
important applications. For example, the polymerization of
propylene, which is identical to ethylene except that one
hydrogen substituent has been replaced by a methyl (CH3)
group, yields "polypropylene" (Scheme 6).

polypropylene

Scheme 6

This material has a higher melting point
(160-170 oC), higher tensile strength, and
greater rigidity than polyethylene. It is also less permeable
than polyethylene to liquids and gases. Polypropylene has many
uses, including synthetic rope, automotive parts like battery
casings, thermal underwear, and the ubiquitous plastic stackable
chairs.

Other important polymers that are
generated from substituted olefin monomers include
polyvinylchloride (PVC) and polystyrene (Scheme 7).

polyvinylchloride

polystyrene

Scheme 7

The chlorine substituent groups on the
PVC polymer chain make it more fire resistant than polyethylene
or polypropylene. They also increase the force of attraction
between individual polymer chains, giving rise to a harder
plastic, which is used in pipes, house siding, toys, and
furniture. A copolymer of vinyl chloride and vinylidene chloride
(CH2=CCl2) is used to make cling
wraps like Saran. The same increase in the force of attraction
between polymer chains that gives PVC its hardness also gives
thin films of Saran its tendency to stick.

Polystyrene is a glassy polymer with
sparkling clarity. It is hard but also rather brittle. Because
polystyrene melts around 90 oC, it can be easily molded into
familiar items likes audio cassette boxes. Interestingly,
"styrofoam" cups are also fabricated from polystyrene.
To make styrofoam, polystyrene beads are mixed with a low-boiling
liquid hydrocarbon, placed in a mold, and heated. As the
hydrocarbon liquid vaporizes, it expands the polymer, causing the
polystyrene to take the shape of the mold. So many bubbles are
trapped in the polystyrene that it becomes a light, opaque foam.
It is also an excellent insulator, which makes it ideal for
applications in the food service industry.

Hands-On Activity:

Using Cochrane's Molecular Models,
make a model of polypropylene. Compare it to a model that your
neighbor has constructed. Are they identical?